1. Field of the Invention
The present invention relates to a flat panel display, and more particularly, to a bonding structure for sealing and bonding an upper substrate and a lower substrate of a flat panel display, a method of forming the bonding structure, and a flat panel display having the bonding structure.
2. Description of the Related Art
A flat panel display can be classified as: a liquid crystal display (LCD), a plasma display panel (PDP), or a field emission display (FED). Recently, a liquid crystal on silicon (LCOS), which forms a thin film transistor (TFT) on a silicon substrate and uses a liquid crystal to display an image, is highlighted due to the demand for a micro-display.
The LCOS panel is a micro-display panel that uses a switching characteristic of the liquid crystal. The LCOS panel is formed to be about one inch by integrating elements of pixels and a switching circuit, and it can realize higher definition than an extended graphics array (XGA) level. Accordingly, the LCOS panel is mainly used as a display in a projection system that expands and projects an image of a display device using an optical lens system. In the projection system using the LCOS panel, a rapid response of the liquid crystal is essential in displaying a moving picture, and especially, when a cell gap on the LCOS panel is reduced, the response time of the liquid crystal is also reduced. Thus, in the LCOS panel, the cell gap should be as small as possible in order to minimize the response time of the liquid crystal. Thus, unlike the general LCD that has a cell gap of 2˜5 μm, the LCOS panel has a small cell gap about 1˜2 μm due to the features of a reflective display device.
However, in a case where the LCOS panel having a fine cell gap is manufactured with respect to the response time of the liquid crystal, an optical interference may occur due to different sizes of cell gaps on the panel, thereby causing a reduction in the image quality of the panel. The above problem is caused by the different thermal properties of an upper substrate and a lower substrate in bonding the upper and lower substrates, or the fine cell gap. Therefore, in order to solve the above problem, it is essential to manufacture the panel to have constant cell gaps.
In the conventional art, in order to maintain a constant cell gap, a spacer may be applied in an active area, a post is disposed on an inter-pixel area, or the spacer may be included in a sealant to maintain a constant cell gap. However, the above methods cause problems in the LCOS panel having small pixels, unlike the LCD. That is, in a case where the spacer is applied on the active area, an arrangement of the liquid crystal is adjusted around the applied spacer, and thus the image may be damaged by the effected liquid crystal. In a case where the post is disposed, the post increases an inter-pixel size, and the panel size also becomes larger. In addition, the quality of the image may be reduced by the orientation defect of the liquid crystal around the post. Also, in a case where the cell gap is maintained by the spacer in the sealant by mixing the spacer in the sealant, it is difficult to compensate for a deformation of the panel, which is caused by the different thermal properties of the upper and lower substrates of the LCOS panel, and thus the cell gap does not have uniformity.
FIG. 1 is a cross-sectional view showing a part of a conventional flat panel display device. As shown in FIG. 1, a sealant 6 is applied between the lower substrate 2 and the upper substrate 4, and the cell gap between the lower substrate 2 and the upper substrate 4 is maintained by the spacer 8 that is embedded in the sealant 6. FIG. 2 is a vertical cross-sectional view showing the conventional flat panel display device. The uniformity of the cell gap shown in FIG. 2 becomes inconsistent when the lower substrate 2 and the upper substrate 4 become sealed to each other.